Coalesce same-sender messages

[gnprice]

When consecutive messages in the message list have the same sender, and there isn't an intervening recipient header (#174) or date separator (#173), we should avoid repeating the sender's name and avatar, much like we do in Zulip web and zulip-mobile. This can significantly smooth the experience of reading a conversation.

This will come after #174, because before that there will always be an intervening recipient header.